The present invention relates to phased array antennas and, more particularly, to a method and apparatus for testing phased array antennas having a plurality of controllable phase shifters on a plurality of subarray module circuit boards.
There exists a type of radar antenna that employs a plurality of subarray module circuit boards mounted in a matrix configuration. Each subarray module circuit board comprises a plurality of radiating dipole antenna elements, a plurality of power dividers, a four-bit phase shifter and a plurality of subarray steering bit networks. Such radar antennas may be used, for example, on the AN/TPQ-37 Firefinder Radar, the Project 548 Air Defense Radar, or the like.
In this type of radar antenna, control circuits for the subarray steering bits of all of the subarray module circuit boards are connected together so that there is no individual control. Consequently, it is very difficult to test the individual subarray steering bits. Typically, testing requires disconnection of the antenna array from the radar system, and disassembly of the radar antenna to perform individual tests on each of the subarray module circuit boards. This means that the testing cannot be performed in the field.
Alternatively, a diagnostic run is made using a near field probe technique. Such a near field probe technique is described in a paper entitled "Near-Field Probe Used As A Diagnostic Tool To Locate Defective Elements In An Array Antenna". The authors are J. J. Lee, Edward M. Ferren, D. Pat Woollen and Kuan M. Lee. The paper was published in the IEEE Transactions on Antennas and Propagation, Vol 36, No. 6, June 1988. In a near field probe, a radio frequency probe moves across a row of dipoles and measures the amplitude and phase response at each position. Due to the close proximity of the array elements, the probe interacts with array elements and causes multiple reflections. As a result, the mismatch of the probe varies from point to point, leading to phase variations as high as 60.degree.. FIG. 5 on page 887 of the above technical paper illustrates these phase variations as high as 60.degree.. The near field probe technique does not provide a good test of the type of subarray module circuit boards under consideration here because the sizes of subarray steering bit phase changes are only 25.degree. or 50.degree.. Since the phase variation of the near field probe technique can be as much as 60.degree., its use can lead to diagnostic errors.
It is an objective of the present invention to provide a method and apparatus for testing all of the subarray steering bits of a radar phased array antenna while the radar is in the field. Another objective of the invention is the provision of a method and apparatus to perform radio frequency tests on all phase shifters of a phased array antenna without disassembling it from a radar system. A further objective of the present invention is to provide a method to isolate radio frequency faults of subarray steering bits without using an expensive near field probe. A still further objective of the invention is the provision of a method for testing subarray steering bits that cannot be tested using current phase toggling techniques.